by TFH Magazine on April 26, 2011 at 11:45 am

By Richard Borowsky

Blind cave fish (front) have no eyes, whereas surface fish (back) of the same species have rather large eyes. Photo by Richard Borowsky

If you want to see the full effects of inter-cave hybridization on eye size, you have to look at very young fish. The eye rudiments of Tinaja and Molino cave fish are much smaller than those of surface fish. Not only are they smaller, they are completely non-functional. In contrast, the eye rudiments of the Tinaja x Molino hybrids are significantly larger than those of their parents. Myquestion was, “Are they large enough and well enough formed to work?”

In order to determine whether any of these fry could see, we placed them individually in a shallow dish in a syrup like substance that made it difficult for them to swim, but did not interfere with their breathing or their eye movements. The dish was surrounded by a vertical cylinder with black and white vertical stripes that we could rotate. When we tested the surface fish, its eyes followed the stripes as they rotated and then, going as far as they could, they would snap back. They would then follow again and snap back once more. This happened repeatedly, until we stopped the movement or reversed the direction of rotation. When we reversed the direction, they would follow in the new direction and snap back in the opposite. The surface fry did this because they could see and because following stripes is an instinct. When we repeated the experiment with either Tinaja or Molino fry, neither of them showed any eye movement at all. The same is true when we used fry from other caves, such as Pachón. Of course, they failed to exhibit the response because they were blind and could not see the stripes.